1. Field of the Invention
The invention relates generally to permanent magnet motors and generators, and particularly to minimizing the cost and cogging torque of such PM machines.
2. Description of Related Art
Due largely to their high torque-to-current and torque-to-volume ratios, permanent magnet (PM) motors and generators are increasingly being used in a wide range of high performance applications such as industrial drives, robotics, computer peripherals, and automotive applications. The PM's are mounted on the rotor, which is the rotating part of the machine, while the stator, which is the stationary part of the machine, contains the coils wrapped around stator teeth that are interconnected via the stator back-iron. Indeed, the combination of stator teeth and back-iron is called the core; the term iron is used throughout to generically denote any material with high magnetic permeability. Often shoes are used with the teeth. As is understood in the art, shoes are widened areas at the teeth tips used to minimize cogging torque by minimizing the discontinuities in magnetic attraction to the rotor magnets. The spaces between teeth are called slots, and the openings to the slots between adjacent shoe tips are called slot gaps.
A Modular (or segmented) stator is one in which the stator is segmented into individual subassemblies that can be subsequently combined to form the overall stator structure. Modular stators per se are not new, but its use to facilitate low-cost coil fixturing is new, as is its use to allow an economical PM machine design, such as set forth in the present invention described in more detail hereinbelow.
The present approach utilizes stator modules that are separately formed and subsequently interconnected to enable cost efficiency by significantly decreasing stator core manufacturing costs, and coil forming and fixturing costs in several ways. (a) First, the modularity greatly reduces the material waste problem because the hollow circular shape of the stator core does not have to be formed in the steel blank when creating the rotor cavity. Instead the shape and cavity are formed only during assembly subsequent to any metal forming. This is particularly important for a ring layout because, by definition, its ring-like shape is extensively hollowed out. (b) Second, the larger more expensive lamination stamping equipment and dies required for forming full stator laminations can be replaced with smaller less expensive dies and equipment for forming the much smaller individual core modules. (c) Third, for the case when each module comprises a single tooth, the coil can be directly wound by a general purpose bobbin winder prior to stator assembly, thereby overcoming the most costly production steps. To adequately insulate the coil from the teeth, rigid or semi-rigid insulation is used on the stator ends, which displaces the coil away from particular module sections while engaging with other insulation sections to remain secured about the teeth.
The invention also utilizes a novel structural support for the stator modules. Besides dovetail connections on each module, rigid pins through the backiron sections of the modules secure them to the frame. Moreover, securing the laminations within each individual stack via a process such as epoxy bonding, welding, cleating, or lamination interlocking via dimples formed within each laminate further enhances rigidity. These connection arrangements can also facilitate a novel lightweight casing, wherein structural support is mainly borne by the endplates, which are directly or indirectly connected to the pins. Unlike other designs, a precision formed cylindrical casing (which becomes significantly more expensive as generator diameter increases) is not needed, and is optional. A spacer ring extends between the stator modules and the endplates to maintain predetermined spacing, and along with the stator modules and endplates completely seal off the machine interior from the elements.
Cogging torque is the torque needed to overcome the tendency of the PM's on the rotor to align at preferential locations opposite the protruding teeth on the stator where magnetic attraction is greatest. Our anti-cogging design, is a natural outgrowth of the modular layout instead of being negatively impacted by it. The highly modular structure makes utilizing the popular anti-cogging technique of skewing essentially impossible. And other anti-cogging techniques such as optimizing the shoe width, the pole width, and the pole-to-teeth number ratio, pairing shoes and poles of different widths, shaping the magnets, notching the teeth and shifting the poles are not sufficient to provide the desired two orders of magnitude lower cogging torque relative to the max generator torque. One aspect of the invention that addresses this challenge is the use of semi-closed (or partially closed) slots, which can decrease the tendency for cogging. Semi-closed slots have slot gap widths that are much smaller than the spacing between similar points on neighboring shoes (i.e. the slot pitch), and for the purposes of this patent we define semi-closed slots as being no wider than 15% of the slot pitch. However, one drawback with such narrow slot openings is that there may not be sufficient spacing to form and fixture the coils around the teeth. Essentially, semi-closed slots make coil insertion through the gap between neighboring shoe tip impractical, thereby requiring the use of needle winders to “sew” the coils around the teeth. One version of the invention that addresses this issue utilizes a single tooth modular structure that allows coil winding directly about the teeth with a simple bobbin winder.
The technique of partially closing the slot is not new, but the invention provides a novel multi-section shoe design that is better able to modulate the magnetic saturation within the shoe, and provide more advantageous magnetic permeability variations for minimizing cogging torque.
To further decrease cogging torque, the invention facilitates using extremely narrow shoe tip gaps of less than 8% of the slot pitch, thereby allowing further cogging torque reduction. For such narrow shoe tip gaps, to decrease excessive magnetic flux leakage between neighboring shoe tips, substantially pointy shoe tips are used, and although this causes substantial magnetic saturation within the tips, the novel multi-section shoe design is better able to modulate the magnetic saturation within the entire shoe, and provide more advantageous magnetic permeability variations for minimizing cogging torque (pointy here is defined as shoe tips with much smaller distal ends than the end that attaches to the stator teeth).
Cogging torque is also minimized through the use of magnets with sloped edges (instead of 90 degree edges). Besides being well-suited for use with semi-closed slots, this technique facilitates a novel mounting arrangement in which pins are used to secure the magnets from translational motion along the surface of the rotor iron, while an adhesive is used along the sloped magnet surfaces and between magnets to constrain against other relative motion. In this way, adhesive between the mating surfaces of the magnets and the rotor iron is not needed and is optional.